LONDON - E.L. James' erotic novel Fifty Shades of Grey, dubbed "mummy porn" by the British media, has become the country's fastest paperback to reach one million sales, trade figures showed.

The story, the first in a trilogy, took just 11 weeks to reach the milestone, 25 weeks faster than the previous record holder, Dan Brown's The Da Vinci Code, according to figures from publishing tracking company Nielsen BookScan.

It also beat its own seven-day record for an adult paperback, shifting 397,889 copies in a week and smashing the 205,130 copies it sold a few days earlier.

Both other titles in the trilogy have also seen sales surge, in one of publishing's biggest stories since J.K. Rowling's seven-book Harry Potter series wound up in 2007. Fifty Shades Darker sold 245,801 while Fifty Shades Freed sold 212,832.

In one week alone, the total of copies sold across the trilogy was around 856,000 copies, or more than twice the number of books sold from the rest of the BookScan top 50.

According to Random House publishers, global sales for the series is in excess of 20 million, with North American sales comfortably topping 15 million.

Fifty Shades of Grey, about a passionate relationship between naïve literature student Anastasia Steele and manipulative entrepreneur Christian Grey, was first published in 2011 and was James' first novel.

Movie rights to the trilogy were bought up by Universal and Focus Features, U.S. media reported in March.

James, a former television executive whose real name is Erika Leonard, lives with her family in London.